I am not sure, but here's my guess. If I say (A != B), people may think of this as being equivalent to saying that there's a property we care about- being not

I am not sure, but here's my guess. If I say (A != B), people may think of this as being equivalent to saying that there's a property we care about- being not equal to B- and we are checking whether A has that property. If I say (Joe != null), then we are saying that there is a property "being not null" and we are checking whether Joe has that property. Feels pretty natural. If I say (null != Joe), then we are saying that there is a property "being not Joe" and we are checking whether null has that property. This is pretty amusing way to think. In the lines after one says something like if (null == myArgument) throw new ArgumentNullException("myArgument") one may proceed with assurance that some properties of null must hold :-) The distinction between null and not-null is so generally useful that I could imagine making a partially-applied function like NotNull, and applying that to Joe (and many other objects). Would NotJoe get much use other than applying it to null? Probably not.